Pressure in fuel tank with engine running

[webcraft]

Just tried to add diesel via can and funnel with the engine running, but it didn't run in freely with the engine running. Goes in fine with engine off.

Breather is clear - I can blow through it and feel air coming out of the tank via the filler.

I could add fuel while running with the old engine, and can do it on Fairwinds, which has a similar engine (Beta 14 instead of 16)

I presume this is the fuel return line slightly pressurising the tank, would probably work with a bigger breather or a wider, less kinked filler pipe.

Not an issue now I know about it, but curious.

- W

 

[Spirit (of Glenans)]

I may well be proved wrong, but it's standard practise to turn off petrol engines when refuelling, so probably similar for diesels.
That aside, yes, if the return pipe is delivering to the tank a larger volume of fuel than the volume of air that can escape through the breather it will result in a small level of pressurisation in the tank. I would see it as a positive phenomenon, as it will aid the flow of fuel to the engine, particularly if it is gravity fed, and the back presure will ensure that no stray water can enter through the filler pipe.

 

[MapisM]

I struggle with this idea, 'cause the return pipe can only throw back in the tank a share of the fuel that the pick-up pipe is sucking out of it.
Ok, theoretically I guess what you guys are envisaging could happen when sucking fuel from one tank and returning it to another.
But it would be silly to run a boat like that, for anything else than temporary fuel transfer purposes.

 

[PaulRainbow]

I'm with Mapis.

With the engine running the fuel tank would be under a slight negative pressure, were it not for the breather pipe, because more fuel is sucked out than is returned. Try blocking the breather and see how long the engine runs for. Something else is going on, though i'm not sure what, perhaps the location of the filler and return pipe within the tank ?

 

[Boater Sam]

Diesel foaming?

 

[Mistroma]

I may well be proved wrong, but it's standard practise to turn off petrol engines when refuelling, so probably similar for diesels.
That aside, yes, if the return pipe is delivering to the tank a larger volume of fuel than the volume of air that can escape through the breather it will result in a small level of pressurisation in the tank. I would see it as a positive phenomenon, as it will aid the flow of fuel to the engine, particularly if it is gravity fed, and the back presure will ensure that no stray water can enter through the filler pipe.

I don't think it's required to turn off diesel or stop smoking when filling even though many people will tell you not to do it. Not a good idea to mop up spillage with a rag and then play a blowtorch on this wick whilst continuing to fill.

I agree with you and everyone else about the return pipe. You need to be pumping fuel in from a different tank to generate pressure. I must admit that the OPs problem is unusual. I wonder if the inlet pipe is long , flexible and vibrating. I imagine that might cause more foaming and reduce flow. Sounds unlikely and it would be interesting to discover the reason.

 

[NorthUp]

"Try blocking the breather and see how long the engine runs for."
Its rare that I would disagree with Paulrainbow, but you might end up with a collapsed tank...

 

[Bilgediver]

"Try blocking the breather and see how long the engine runs for."
Its rare that I would disagree with Paulrainbow, but you might end up with a collapsed tank...

Not a good idea. The spill reyurns are from the HP side of the injectors and fuel pumps and the spill line might have rubber or neoprene sections which would burst if the spill was blocked up at the tank end.

 

[PaulRainbow]

"Try blocking the breather and see how long the engine runs for."
Its rare that I would disagree with Paulrainbow, but you might end up with a collapsed tank...

No, you won't, not even if the tank is plastic. Fuel is sucked from the tank by the low pressure lift pump, it isn't capable of collapsing a tank.

 

[Spirit (of Glenans)]

Could the returned fuel be warm enough to cause the air in the tank to expand? If this is so, would the pressurising effect be more noticeable with an almost full tank or an almost empty one?

 

[PaulRainbow]

Not a good idea. The spill reyurns are from the HP side of the injectors and fuel pumps and the spill line might have rubber or neoprene sections which would burst if the spill was blocked up at the tank end.

I didn't say "block the return", i said "block the breather" and it wasn't actually a suggestion to really, deliberately. block the return.

The return line is relatively low pressure anyway, a lot of these old tech diesels have rubber spill pipes that just push onto the top of the injectors.

 

[prv]

I wonder whether some quirk of vibration and harmonics, rather than pressure, was stopping the fuel flowing down the pipe smoothly?

Pete

 

[AntarcticPilot]

I may well be proved wrong, but it's standard practise to turn off petrol engines when refuelling, so probably similar for diesels.
That aside, yes, if the return pipe is delivering to the tank a larger volume of fuel than the volume of air that can escape through the breather it will result in a small level of pressurisation in the tank. I would see it as a positive phenomenon, as it will aid the flow of fuel to the engine, particularly if it is gravity fed, and the back presure will ensure that no stray water can enter through the filler pipe.

The common scenario for doing what @webcraft suggests is refueling under way, in which case you probably don't want to turn the engine off!

 

[fisherman]

Tank baffle, engine feed one side, return the other, and slow drain through it? It's a long shot......

 

[Deleted member 36384]

Just tried to add diesel via can and funnel with the engine running, but it didn't run in freely with the engine running. Goes in fine with engine off.

Breather is clear - I can blow through it and feel air coming out of the tank via the filler.

I could add fuel while running with the old engine, and can do it on Fairwinds, which has a similar engine (Beta 14 instead of 16)

I presume this is the fuel return line slightly pressurising the tank, would probably work with a bigger breather or a wider, less kinked filler pipe.

Not an issue now I know about it, but curious.

- W

Has the fuel return pipe been modified to tie into the fuel tank fill up line, maybe low down. If so the extra fuel flow into he tank fill up line could be restricting any fuel added when the engine is running. Also, is the return flow from the fuel pump spraying over the fuel tank fill up inlet at the tank, it could cause a similar issue. Its a common issue on pumping systems when the inlet and dump returns are too close - different arrangement but similar issue.

The hotter returning fuel could heat up the tank such that you are feeling the warm air venting through the tank fill up line, which is being replaced by colder air through the tank.

Are both the vent line and fill up hose stiff and not kinked or collapsing (highly unlikely) when the engine is underway and introducing restrictions?

Do let us know if you find a solution.

 

[prv]

Are both the vent line and fill up hose stiff and not kinked or collapsing (highly unlikely) when the engine is underway and introducing restrictions?

Good thought, and maybe not as unlikely as you think. I once had a raw water hose installed with a twisting force along it. When cold, it held its shape fine and there was no sign of the torque imposed, but when the compartment heated up after running the engine for a while, the rubber softened and the hose twisted shut. If the OP’s filler hose runs through the engine bay, it’s possible that something similar could be happening. Not likely, but we seem to have exhausted the likely possibilities already.

Pete

 

[Mistroma]

Are both the vent line and fill up hose stiff and not kinked or collapsing (highly unlikely) when the engine is underway and introducing restrictions?

Do let us know if you find a solution.

I said in an earlier post:
"I wonder if the inlet pipe is long , flexible and vibrating. I imagine that might cause more foaming and reduce flow."

The more I think about it the more it seems likely to be the inlet hose itself. Flexible or stiff and vibrating to cause foaming or more likely collapsing.

It might have delaminated inside and the flap only restricts filling when vibration causes it to flop outwards slightly. I had that once with an exhaust hose and the filler hose might also be spiral wound.

Feeding the return back to the filler or hot fuel seem really unlikely.

My current preference is the filler hose and possibility of a fault (e.g. de-lamination).

 

[webcraft]

The filler hose is only about 3" long.

- W

 

[Mistroma]

The filler hose is only about 3" long.

- W

Well that kills all my hose related theories. How narrow is the filler and what is the hose diameter? I doubt the information will help much. Filling a tank through a normal size of filler with only 3" of hosepipe is getting quite close to pouring diesel into a bucket.

I'm struggling to think what would cause the issue you describe. You must be able to see into the tank and would spot any obstructions.

 

[webcraft]

I can't see into the tank because the hose is bent in two dimensions (but not kinked)

Am dead chuffed though that there is pressure in the tank when I blow down the breather, at least that is working.

- W